Terahertz waves consist of electromagnetic waves having a frequency of about 0.01 to 100 THz between light waves and radio waves, and have an intermediate property between light waves and radio waves. In recent years, techniques of acquiring information of an object by detecting terahertz waves having passed through the object or by detecting the state of terahertz waves reflected by the object have been proposed (see, for example, PTLS 1 to 3).
PTL 1 discloses a reflection type detection apparatus which acquires information of an object by detecting the state of terahertz waves reflected by the object. The detection apparatus disclosed in PTL 1 includes a terahertz wave generation section, a prism and a terahertz wave detection section. The terahertz wave generation section applies femtosecond pulse laser light to InAs to generate terahertz waves. The terahertz waves are incident on the prism through a light path including two off-axis parabolic mirrors. The object is placed on a planar surface of the prism. When totally reflected by the planar surface under the object, the terahertz waves incident on the prism become terahertz waves containing the information of the object. The terahertz waves containing the information of the object are emitted from the prism, and reach the terahertz wave detection section through the light path including the two off-axis parabolic mirrors. The terahertz wave detection section detects the terahertz waves containing the information of the object.
In the detection apparatus disclosed in PTL 1, a large number of optical elements are provided between the terahertz wave generation section and the prism, and between the prism and the terahertz wave detection section. As such, the detection apparatus disclosed in PTL 1 has a problem that the device size is large. In addition, since terahertz waves are absorbed by the moisture in the air, the space between the terahertz wave generation section and the prism and the space between the prism and the terahertz wave detection section are required to be filled with nitrogen, or vacuumized. To solve such problems, PTL 2 proposes a technique in which the terahertz wave generation element and the terahertz wave detection element are integrated with the prism.
PTL 2 discloses a reflection type detection apparatus which acquires the information of an object by detecting the state of the terahertz waves reflected by the object and a reflection type detection device used for the reflection type detection apparatus. The detection apparatus disclosed in PTL 2 includes a light source, a detection device, and a light detector. The detection device includes a prism, a terahertz wave generation element disposed on the incidence surface of the prism, and a terahertz wave detection element disposed on the emission surface of the prism. The light source applies femtosecond pulse laser light to the terahertz wave generation element of the detection device. As a result, terahertz waves are generated at the terahertz wave generation element, and the terahertz waves travel in the prism. The object is placed on the planar surface of the prism. When totally reflected by the planar surface under the object, the terahertz waves incident on the prism become terahertz waves containing the information of the object, and reach the terahertz wave detection element. The terahertz wave detection element generates light containing information of the object in accordance with the input terahertz waves. The light detector detects the light containing the information of the object.
In addition, PTL 3 discloses a transmission type detection device which acquires information of an object by detecting the state of the terahertz waves having passed through the object. FIG. 1 is a perspective view of the detection device disclosed in PTL 3. As illustrated in FIG. 1, detection device 10 disclosed in PTL 3 includes two metal plates 12a and 12b, two polystyrene plates 14a and 14b and two photoconductive antennas 16a and 16b. Two metal plates 12a and 12b are disposed to face each other with a distance of approximately 100 μm therebetween, and two polystyrene plates 14a and 14b are disposed between metal plates 12a and 12b. The laminated body composed of two metal plates 12a and 12b and two polystyrene plates 14a and 14b serves as a parallel flat plate waveguide path. Space 18 for housing an object is formed between two polystyrene plates 14a and 14b. The distance between two polystyrene plates 14a and 14b is approximately 50 μm. The object housed in space 18 is thus present at a middle point of the waveguide path. Photoconductive antenna 16a is disposed at one end portion of the laminated body, and photoconductive antenna 16b is disposed at the other end portion of the laminated body. When femtosecond pulse laser light is applied to photoconductive antenna 16a, terahertz waves are generated. The terahertz waves travel through polystyrene plate 14a, space 18 (object) and polystyrene plate 14b, and reach photoconductive antenna 16b. Photoconductive antenna 16b detects the terahertz waves transmitted through the object (converts the terahertz waves into an electric signal).
Since attenuation of terahertz waves is large, and terahertz waves are difficult to be directly applied to polystyrene plate 14a, detection device 10 disclosed in PTL 3 applies laser light to photoconductive antenna 16a to generate terahertz waves. In addition, since terahertz waves are attenuated when the size of space 18 is increased, the size of space 18 is preferably small as much as possible.
Even with a small size, detection device 10 disclosed in PTL 3 can detect the state of the terahertz waves having passed through the object.